Thermosetting resins, particularly unsaturated polyester resins containing fillers and reinforcements have been used for premix, transfer molding, compression molding, injection molding, and mat and preform molding. Although polyester resins generally perform satisfactorily in the above-molding processes, the resins are subject to several deficiencies. Parts molded from polyester resins are subject to shrinkage and warpage upon cooling, and often the molded product exhibits surface waviness, roughness and sink markings, especially when the articles have relatively intricate shapes and sections of varying thickness.
Various suggestions have been made in the art for improving the surface characteristics (also called low profile characteristics) of polyester resins. The automotive industry's need for thinner, smoother and tougher low profile sheet molding compound (SMC) composites for body panel parts has created an even greater interest in overcoming the shrinkage problems of thermosetting resins such as polyester resins. One proposed solution to the shrinkage problem is the inclusion of additives in the polyester resin which are believed to expand as the polyester resin is cured thereby minimizing and/or offsetting the shrinkage of the polyester. Such additives are generally referred to in the art as "low profile additives" since they improve the low profile characteristics of the cured polyester resin. In other words, low profile additives have been added to unsaturated polyester compositions for the purpose of obtaining compositions which can be molded to thermoset articles having surfaces which truly reflect the surfaces of the mold.
A number of the low profile additives which have been suggested for improving the surface properties of polyester resins are thermoplastic resins such as polymethylmethacrylates, polystyrenes, and other polymers derived from compounds containing a polymerizable olefinic group. U.S. Pat. No. 3,503,921 describes a polyester premix containing polystyrene and monomeric styrene. These compositions are described as providing a polyester resin system which, when molded exhibits relatively low shrinkage, non-warp, and improved surface characteristics.
U.S. Pat. No. 3,674,893 describes the modification of an unsaturated vinyl ester thermosetting resin by inclusion of a copolymerizable monomer and unsaturated polydiene rubbers such as styrene-butadiene copolymers. Such resin mixtures are reported to provide improvements in one or more properties of impact strength, stability, paintability, surface smoothness and uniformity of pigmentation in reinforced molded articles prepared therefrom.
U.S. Pat. Nos. 3,701,748 and 3,772,241 describe unsaturated polyester resin compositions containing a monomer having an olefinic group copolymerizable with the unsaturated polyester, and a thermoplastic polymer which is soluble in the monomer, but when present during the copolymerization of the unsaturated polyester in the monomer, yields an optically heterogeneous cured composition. It is reported that through the use of the specific combinations of the three components, the overall polymerization shrinkage is at most very low. Fibrous reinforced articles prepared from such modified unsaturated polyesters are characterized by smooth surfaces.
U.S. Pat. No. 3,718,714 also relates to unsaturated polyester compositions containing a thermoplastic polymer. In this patent, the thermoplastic polymer is a polymer of vinyl acetate having an average of at least one carboxyl group per molecule. Such modified compositions have particular utility in sheet molding applications to form thermoset articles characterized by excellent impact strength and by excellent surface properties.
The use of low profile additives for improving the surface characteristics of cured thermosetting resins such as unsaturated polyester resins is not problem-free. One of the difficulties which arises from the preparation of a mixture of a thermosetting resin such as the polyester resin and a thermoplastic resin composition is that the thermosetting resins and the thermoplastic resins generally are immiscible and result in the formation of a two-phase system. Although techniques have been developed in the art for satisfactorily handling two-phase systems, particular problems have been observed when it is desired to prepare colored or pigmented molded articles from such two-phase systems. Normally, color pigments are utilized to produce the desired color to the molded article. In these types of systems, color pigments appear to have a different affinity for thermosetting resins than the thermoplastic resins, and thus, the distribution of the color pigments in the molded articles is not always as uniform as desired. Also in the finished product, there is a haze caused by the differences in the index of refraction of the different resins.
U.S. Pat. No. 4,555,534 describes a process for improving the uniformity of pigmentation in thickenable low shrink polyester molding compositions. The improved pigmentation in the thickenable polyester molding compositions containing a carboxylated vinyl ester polymer low profile additive is achieved either by incorporating a surfactant in the composition, by using as the low profile additive, a vinyl acetate/maleic acid copolymer, or by a combination of the two methods. A wide variety of surface active compounds are disclosed as being useful, and as a general rule, the ionic surface active compounds are reported as giving the best results.